I am trying to find the way to arrive at a suitable electrolytic capacitor that would serve the purpose of retaining the powersupply at the working voltage level for a predefined time. I am not successful in making use of the t=RC, or the E =0.2C.V.V , or the E = I.I.R.t where t is the time for which I wish to have the voltage remain at the working level, R the load resistance, C the electrolytic capacitor, V the DC voltage, I the load current.

Say I want to achieve a 100ms of retention of the voltage from the instant of the source shutdown. The attached ckt dgm, wavefm dgm should give you an idea i wish to simulate.

Request your comment in where in my calculation and in the simulation part I am commiting mistake.

If you are trying to maintain some voltage level across a load sourced only buy a capacitor, then you are going to be furstrated. As soon as the capacitor has a lod drawing current, the voltage will start to decrease. You can delay the rate of decrease by increasing the capacitance, but the voltage will always decrease as current is drawn from the capacitance.

Your best best is to keep the regulator in circuit, with enough voltage on the capacitor/s to supply the desired voltage output through the regulator for the desired period.